Check out Gaku Homm's book, the Structure of Aikido. It lays out relations between sword vs. sword, empty hand vs. sword, and mpty hand vs. empty hand techniques. The theme of the book is that they're all basically the same movements, and who has what is fairly inconsequential.

Once you get a hold of a copy, look for the open hand technique that most resembles the tachidori you're thinking of, and the tachidori will be on the same page, right next to it.